TV transmission is becoming less analog and more digital. To support digital broadcasting, set-top boxes (STB) are needed to decode the received broadcast stream so the signal can be shown on the display. Motion Picture Engineering Group (MPEG) standards specify that digital TV (DTV) transmissions are clocked using a time reference embedded in the digital stream. This time reference is used by the STB to synchronize the STB local clock on the system clock. This method is called the program clock reference (PCR) method. Alternatively, MPEG standards allow clock synchronization by using the video buffering information encoded into the stream, referred to as the video buffer verifier (VBV) method.
The ability to display a DTV broadcast without degradations such as video freeze, blocks, audio cuts, and so forth, can be an important distinguishing characteristic in the marketplace. In particular, the ability of the STB to decode non-compliant streams can impact a customer's buy-decision, particularly in countries where the broadcast conditions do not accurately follow DTV standards. In the PCR method, if the system clock reference is not correctly encoded in the stream, or is not correctly decoded by the STB, then the synchronization between the DTV broadcast and the STB can be lost, resulting in the STB local clock being faster or slower than the system time. The main consequence is that the DTV bit stream will not be decoded by the STB at the same speed it has been broadcasted by the system. Under these conditions, the STB's internal buffer will eventually either run out of data or overflow, leading to artifacts like video freeze, macro block display, and audio cuts. The VBV method can be unreliable as an alternative because very often the VBV data is not present in the DTV stream.